Rules about parking across dropped kerbs/driveways generally seem to be pretty unclear and are the subject of much Internet hearsay. Some people say it's not an offence at all (even if the driveway is in use) whilst others say it's only an offence if you prevent someone accessing the public highway (i.e. driving out of their property onto the road, but not the other way around.) Whatever the circumstances here, the PCN (Penalty Charge Notice) should provide details of how to appeal, assuming it's one issued by or on behalf of the council and not by some dubious private parking enforcement firm. If it's the former the appeal process should be free so I suggest you go ahead by providing the basic facts as you've described them above. My hunch is that you will be successful as it doesn't appear from your description that you were actually causing an obstruction or contravening any rules. Anyway, it doesn't hurt to try! Don't leave it too long to appeal though so you don't risk incurring the full charge rather than the discounted rate. Several people on my road park over their own driveways, often because modern cars are a little too big to fit easily (if at all) and, to my knowledge, none of them has ever received a ticket.